Eighteen Years of Respiratory Syncytial Virus Surveillance

Changes in Seasonality and Hospitalization Rates in Southwestern Alaska Native Children

Dana J.T. Bruden, MD; Rosalyn Singleton, MD, MH; Carolyn S. Hawk, MD; Lisa R. Bulkow, MS; Stephen Bentley, BS; Larry J. Anderson, MD; Leslie Herrmann, MD; Lori Chikoyak, RN; Thomas W. Hennessy, MD

Disclosures

Pediatr Infect Dis J. 2015;34(9):945-950. 

In This Article

Materials and Methods

The study population has been described elsewhere.[8,20] Briefly, the YKD covers an area of 195,000 km2 in southwestern Alaska. It is home to ~25,000 primarily (>85%) Yupik Eskimos, who live in Bethel (population approximately 6000) or 49 small villages.[22] Travel is by small aircraft, snow machine or river boats, as there are no roads between most villages. The YKD birth cohort, obtained from birth records, shows an average of 635 babies born annually.

Methods for RSV surveillance in YKD have been previously described.[9] This surveillance for RSV has been conducted since late 1993 to present. The first 3 years were active surveillance; a nasopharyngeal aspirate was obtained from children younger than 3 years of age hospitalized with LRTI. Since October 1996, we obtained hospitalization data, including RSV test results, from electronic medical records of YKD Regional Hospital (YKDRH) and the referral hospital in Anchorage Alaska (Alaska Native Medical Center).[9] RSV testing from 1994 to 1996 was done by rapid antigen enzyme immunoassay test pack (Abbott, Oak Park, IL) or by culture and direct immunofluorescence assay (Bartels, Issaquah, WA) on nasopharyngeal aspirate samples.[12] Rapid antigen testing was done by Directogen RSV (Becton Dickson, Cockeysville, MD) during 1996–2004[12] and by Binax Now (Inverness Medical; Princeton, NJ) during 2005–2012.[10,23] The percentage of <1 year old hospitalized LRTI patients tested for RSV was similar between all study years.

Hospitalization data (admission and discharge dates, ICD9 discharge diagnoses, village of residence, birth date, RSV test results and dates) were obtained for all YKD children younger than 3 years of age. Hospitalizations were merged if a child was readmitted within 3 days of a previous hospitalization's discharge date. Data from a child transferred from YKDRH to Alaska Native Medical Center were combined into 1 hospitalization. The surveillance was approved by the Alaska Area Institutional Review Board and health boards from YK Health Corporation, South central Foundation and Alaska Native Tribal Health Consortium.

For calculation of RSV rates for each season, July 1–June 30, only the first positive hospitalization in children younger than 1 year of age was used, and rates are reported per 1000 infants. We calculated season onset, offset and peak week, using the methodology of Mullins et al[13] and required that >5 RSV tests were performed during the peak week. The length of the RSV season was calculated as the number of weeks from the onset week through the offset week.

Weather data were obtained from the Alaska Climate Research Center, University of Alaska Fairbanks. We selected 6 locations geographically spread throughout YKD coast and inland that had complete daily weather records during the surveillance years of interest, shown in Figure 1. We looked at the long-term relationship between temperature and RSV hospitalizations through use of an autoregressive integrated moving average model. The dependent variable was the monthly rate of RSV hospitalization for YKD. To assess the lag between temperature fluctuations and RSV, we ran a cross-correlation analysis. Finally, to investigate year-to-year variation in the RSV rates and timing of the season, we calculated the average temperature and dew point during the winter months (October–May) and during the fall months (October–November). We tested for an association between temperature and dew point and the season's severity (RSV rate) and onset by use of Spearman's rank order statistic.

Figure 1.

Map of study region in Alaska along with locations of villages, subregional clinics and regional hospital.

In a geographic analysis, we calculated the RSV rate for each of 49 villages of residence for children younger than 1 year of age for 2 periods: July 1995–June 2005 and July 2005–June 2012. For each village, we obtained: (1) the percent of households that lack complete plumbing, (2) the percent of families below the poverty level, (3) the average number of persons per room, (4) the percent of households with >1.5 persons per room and (5) the percent of households using wood as a heat source.[22] For the 1995–2005 period, we obtained all data from the Census 2000 and for the 2005–2012 period, we obtained data from the Census 2010 and the American Community Survey.[22] Additionally, because a major determinant of winter weather conditions in Alaska is proximity to the ocean, we examined whether the community was geographically situated on the ocean coast. Two other variables were examined, at the village level, related to healthcare access: the presence of a subregional clinic and the distance to YKDRH. Univariate and multivariate analyses were conducted by use of Poisson regression. We calculated the percentage of the RSV decline accounted for by socio-economic factors by comparing the relative rates (RR) in the univariate and multivariate adjusted models. Variables were selected in the multivariable model through use of a purposeful backwards elimination. Variables were considered confounders and remained in the model if their exclusion changed the value of other coefficient(s) of interest by more than 15%. P values less than 0.05 were considered statistically significant.

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